The aforementioned applications have in common that they require a large amount of air to be filtered with high particle filtration efficiency. In some applications a single cassette filter has the capacity for filtering more than 1000 m3 per hour, with a typical filter size of 600 mm×600 mm×300 mm filtering about 2,500 m3 to 5,000 m3 per hour. Many cassette filters may be used in parallel in order to filter an amount of air of more than 10,000 m3 per hour or even more than 50,000 m3 per hour. In such applications, cassette filters or cassettes are mounted in a partition which separates an upstream volume generally referred to as the “dirty air section” from a downstream volume generally referred to as the “clean air section”. The partition may be in the form of a wall with openings in which the cassette filters are mounted or may be in the form of a rack defining a plurality of openings in which the cassette filters are mounted so as to create a substantially airtight partition between the dirty air and clean air sections.
Many installations have limited space on the clean air section of the filter unit due to obstructions or installed equipment. Such constraints limit the depth to which the cassette filter can extend into the clean side, and has therefore limited the size of cassette filters. The size limitation limits the amount of media that can be installed in a filter. This problem is particularly acute when retrofitting existing filter systems with high efficiency media. High efficiency media often has increased pressure drop, making it particularly important to maximize filter area when retrofitting existing media.
Cassette filters typically comprise a plurality of filter panels arranged so that pairs of panel filters form V-shapes extending from the cassette filter's upstream end to the cassette filter's downstream end. Each filter panel is composed of multiple pleats of filter media extending generally parallel to the overall filtration path, so that air or gas to be filtered passes through the pleats in a generally straight manner. The filter panels are mounted in a casing and are sealed to a mounting frame at one end of the filter. The mounting frame provides a mounting face for mounting the cassette filter to a corresponding mounting face of the partition so that the cassette filter extends into and through the opening of the partition into the clean air section. Filter cassettes may be accessed from the dirty air section side and may be easily removed and replaced.